1. Field of the Invention
The present invention generally relates to fluid circulation operations for a wellbore. More particularly, the invention relates to a plug and ball seat assembly for releasing a wiper plug during cementing operations.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. The well is then drilled to a second designated depth, and a second string of casing, or liner, is run into the well. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second “liner” string is then fixed or “hung” off of the surface casing. Afterwards, the liner is also cemented. This process is typically repeated with additional liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
In the process of forming a wellbore, it is sometimes desirable to utilize various plugs. Plugs typically define an elongated elastomeric body used to separate fluids pumped into a wellbore.
Plugs are commonly used, for example, during the cementing operations for a liner. During a cementing operation for a liner, the liner is typically run into the wellbore at the end of a run-in string. The run-in string usually comprises a string of drill pipe. At the same time, a liner wiper plug is run into the wellbore at the lower end of the drill pipe. The run-in string places the liner and the wiper plug at the desired depth in the wellbore. After a wiper plug is released, it is pumped downhole in order to clean or “wipe” the inside of the liner of mud and debris. In this respect, the liner wiper plug employs radial wipers to contact and wipe mud from the liner as the plug travels downhole.
The process of releasing a liner wiper plug downhole is typically accomplished by pumping a drill-pipe dart down the drill string. The dart is pumped downward by injecting cement or other desired circulating fluid into the wellbore under pressure. The fluid forces the drill pipe dart downward into the wellbore until it contacts a seat in the wiper plug at or near the base of the drill pipe. The dart sealingly lands into the wiper plug. Hydraulic pressure from the injected fluid ultimately causes a releasable connection between the wiper plug and the drill pipe to release, thereby allowing the dart and the wiper plug to be pumped downhole as a single plug. This consolidated wiper plug separates the fluid above the plug, such as cement, from the drilling mud or other circulating fluid below the plug.
As noted, wiper plugs are retained through a releasable connection. A variety of mechanisms have been employed to retain and subsequently release liner plugs. These include mechanisms such as shear screws, dogs, collets, and retaining rings. Many of these utilize a sliding sleeve that is held in place by a shearable device. When the drill-pipe dart lands in the sliding sleeve, the shearable device is sheared and the sleeve moves down, allowing the plug to release.
Certain disadvantages exist with the use of the above release mechanism arrangements. For example, during well completion operations, the release mechanism, such as the shear screws or the sliding sleeve, is subjected to various stresses which may cause premature release of the wiper plug. In some situations the sliding sleeve is subjected to an impact load by a ball or other device as it passes through the inside of the plug. In other situations, a pressure wave may impact the releasable connection. In either of these situations, it is possible for the sliding sleeve or other release mechanism to shear and to thereby inadvertently or prematurely release the wiper plug.
There is a need, therefore, for a more effective release mechanism for a wiper plug. There is a further need for a releasing mechanism for a liner wiper plug which will not prematurely release the plug in the event of a pressure surge. Still further, there is a need for a liner wiper plug releasing mechanism which does not rely upon a sliding sleeve.